Hindered amine light stabilizers (HALS) based on diesters of bis(substituted-piperidine) are known and are prepared by various methods. One method of preparation is transesterification of a dicarboxylic acid ester by reacting a dicarboxylic acid ester, such as dimethylsebacate, with a substituted-4-hydroxypiperidine in the presence of a catalyst, such as an alkali metal amide, in the absence or presence of an inert solvent, such as toluene, to form the diester of the piperidine alcohol employed and methanol as the by-product.
For example, U.S. Pat. No. 4,021,432 describes the transesterification of dicarboxylic acid esters in the absence or presence of an inert solvent, such as benzene, toluene or xylene, with a substituted piperidinol compound in the presence of a transesterification catalyst, an alkali metal amide such as lithium amide, whereby the alcohol produced in the reaction is removed. However, the total reaction time is from about 4 to about 7 hours.
In U.S. Pat. No. 4,461,898 mixtures of esters of polyalkylpiperidine derivatives are prepared by reacting 2 mols of a Piperidinol with 0.9 to 1.3 mols of a diester, in the melt, at between 100.degree. and 145.degree. C., in the presence of an alkali metal amide, preferably lithium amide, as a catalyst. After the reaction has started, the alcohol formed is removed by distillation, first at atmospheric pressure, and then under vacuum for an additional 2 to 3 hours.